The detection of biological species is an important analytical technique in several industries, including the food industry, environmental monitoring, and health care, e.g. for the prevention of post-surgical infection. For example, traditional methods of detecting bacteria require the culturing of microorganisms. While these methods provide a desired level of sensitivity, they require highly trained laboratory personnel and typically need days to obtain results.
Post-surgery infections constitute the most common infection for surgical patients. In many cases, bacteria still remain on the skin after the skin has been prepared for surgery. About 20% of the population have high bacteria counts on the skin, i.e. more than 1000 CFU cm−2 and represent those who are at greatest risk of infection. Accordingly, microbial detection is particularly important when preparing these patients for surgery. The bacteria on the skin are typically of the Gram positive type, and so it is important to be able to distinguish Gram positive bacteria. In other health care situations, it is important to differentiate between Gram negative and Gram positive bacteria, and to detect the total bacterial number from skin, wound, and body fluid (e.g., urine) samples. It is important also to be able to detect bacteria from environmental samples taken from food processing and clinical settings. The types of bacteria in these samples are typically Gram negative. Gram negative bacteria can be detected at very low concentration with a stable limulus amoebocyte lysate (LAL) reagent. The chromogenic substrate (p-nitroaniline) supplied with the commercial LAL changes from colorless to yellow in the presence of lipopolysaccharide (LPS) and is readily measured in a spectrophotometer. LAL cannot, however, distinguish between bacteria-surface bound LPS and free (soluble) LPS in solution. Free LPS is present in most environmental samples. It is either released under normal bacterial growth processes or from the dead gram-negative bacteria. The separation of soluble LPS from intact bacteria is a very challenging issue.